1. Field of the Invention
The present invention relates to a band eliminate filter suitable for use in, for example, a high-frequency high-power system, and a communication apparatus including the band eliminate filter.
2. Description of the Related Art
Band eliminate filters for use in a high power system include an example as disclosed in Japanese Unexamined Patent Application Publication No. 11-274817 in which a waveguide and a cavity resonator are used. As shown in the above Publication, a problem in a filter for use in a high power system is discharge (arc discharge) at high power (high voltage). Also, recently, it is common that band eliminate filters are used in base stations for mobile communication. This case not only obviously requires measures for high power (high voltage), but also requires a filter having a very low loss in the vicinity of an attenuation range because of proximity of operating bands in recent years.
Although it is considered that the invention in Japanese Unexamined Patent Application Publication No. 11-274817 has durability against power and a low loss since the waveguide and the cavity resonator are used, the invention has a problem in that filter size is very large.
In addition, as shown in Japanese Unexamined Patent Application Publication No. 04-188902, Japanese Unexamined Utility Model Application Publication No. 06-066103, and Japanese Unexamined Patent Application Publication No. 02-034001, in each of commonly invented band eliminate filters, a dielectric resonator is used as a resonator, microstrip lines formed by a dielectric substrate are used as transmission lines, and a plane chip capacitor or a distributed-constant capacitor formed on the substrate is used as a capacitor. Although this type of band eliminate filter can be reduced in size, it has a possibility that many small gaps between electrodes may discharge at high power (high voltage), and the microstrip lines generally have large loss, thus causing a deterioration in insertion loss. Also, the chip capacitor and the capacitor formed on the microstrip lines cause a deterioration in insertion loss of a passband in the vicinity of an attenuation range since the capacitors each have Q.
When the passband is very close to the attenuation range, a reflection characteristic (return loss) in the vicinity of the attenuation range must be improved. For example, in the case of generating a return loss peak in the vicinity of the lower side of the attenuation range, the capacitance of the capacitors must be reduced. Due to the required characteristic, when the capacitance of the capacitors is very small, the use of the dielectric plane capacitor and the chip capacitor greatly reduces the size, so that assembly is difficult. In the case of the small size, differences in dimension precision of electrodes, dimension precision of dielectric material, and dielectric constant appear as a change in capacitance. Thus, a difference easily occurs in characteristics, which requires adjustment. Similarly, a difference in assembly easily appears as a change in capacitance, thus causing a difference in characteristics. For example, when a capacitance of 0.5 pF is obtained by a dielectric chip capacitor having a dielectric constant of 21 and a thickness of 1 mm, the shape of the chip capacitor is square, having each side of 1.63 mm. In this case, only a minute change of 0.05 mm in one side causes a 5-percent change in capacitance. Similarly, a change of 0.05 mm in thickness also causes a 5-percent change in capacitance. The 5-percent change generates a change of approximately 15 MHz in the return loss peak and a change of approximately 12 MHz in the attenuation peak. Also, since the case of adjusting the capacitance of the capacitors requires very high processing precision, a lot of experience is required.
FIGS. 8A and 8B show examples of characteristics of a band eliminate filter in which one stage of a resonator is coupled to a transmission line. FIG. 8A shows transmission characteristics S21 and S11 in a case in which the capacitance of the capacitor of the band eliminate filter is 0.290 pF, and FIG. 8B shows the transmission characteristics S21 and S11 in a case in which the capacitance of the capacitor of the band eliminate filter is 0.387 pF. In both cases, identical component values are used, except for the capacitance.
As shown in FIGS. 8A and 8B, when the capacitance of the capacitor for coupling the resonator with the transmission line changes only approximately 33 percent, the central frequency of the elimination band greatly changes from 1994.75 MHz to 1936.81 MHz. Also, an increase in the capacitance of the capacitor lowers the peak of the attenuation range, but increases the distance between the return loss peak and the peak of the attenuation range.